This invention relates generally to coordinate position digitizing system adapted to be employed in apparatus for displaying and creating figures in which a tablet and a stylus used for drawing figures on the tablet are provided and a position of the stylus on the tablet is detected to produce data representing a figure drawn by the stylus on the tablet, and more particularly, to a coordinate position digitizing system in such a type as mentioned above in which a position of the stylus on the tablet is detected electrostatically.
There has been proposed a coordinate position digitizing system having a tablet and a stylus used for drawing figures on the tablet, in which a position of the stylus on the tablet is detected electrostatically with the configuration as shown in FIGS. 1 and 2. Referring to FIGS. 1 and 2, a position determining plate 10 which forms a tablet comprises, as shown in FIG. 2, a first transparent insulating layer 11, a plurality of strip electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M arranged on the first transparent insulating layer 11 to extend parallel to one another at regularly spaced intervals, a second transparent insulating layer 12 covering the surface of the first transparent insulating layer 11 on which the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M are disposed, a plurality of strip electrodes X.sub.1, X.sub.2, - - - X.sub.N arranged on the second transparent insulating layer 12 to extend parallel one another at regularly spaced intervals in the direction perpendicular to the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M, and a third transparent insulating layer 13 covering the surface of the second transparent insulating layer 12 on which the electrodes X.sub.1, X.sub.2, - - - X.sub.N are disposed. On the position determining plate 10 thus provided, a detecting electrode 20 which forms a stylus is placed. Each of the electrodes X.sub.1, X.sub.2, - - - X.sub.N which are located on the surface closer to the detecting electrode 20 than the surface on which the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M are located has the width narrower than that of each of the eletrodes Y.sub.1, Y.sub.2, - - - Y.sub.M. The detecting electrode 20 is grounded through a predetermined constant capacitance C.sub.L.
During a predetermined period, switches 32.sub.1, 32.sub.2, - - - 32.sub.N are supplied successively with pulses from corresponding output terminals N.sub.1, N.sub.2, - - - N.sub.N of a shift register 31 to be turned to respective terminals B thereof from respective terminals A thereof, so that the electrodes X.sub.1, X.sub.2, - - - X.sub.N are supplied successively with the positive voltage V.sub.CC of a voltage source +V.sub.CC during each period of the pulse from the shift register 31. During another predetermined period, similarly, switches 34.sub.1, 34.sub.2, - - - 34.sub.M are supplied successively with pulses from corresponding output terminals M.sub.1, M.sub.2, - - - M.sub.M of a shift register 33 to be turned to respective terminals B thereof from respective terminals A thereof, so that the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M are supplied successively with the voltage V.sub.CC of the voltage source +V.sub.CC during each period of the pulse from the shift register 33.
Under such conditions, a voltage V.sub.O at the detecting electrode 20 is represented by the following equation: ##EQU1## where C.sub.P stands for a capacitance obtained between the detecting electrode 20 and one of the electrodes X.sub.1, X.sub.2, - - - X.sub.N or Y.sub.1, Y.sub.2, - - - Y.sub.M to which the voltage V.sub.CC is supplied. According to this equation, it is understood that the voltage V.sub.O is the higher, the larger the capacitance C.sub.P is, that is, the shorter distance to the electrode supplied with the voltage V.sub.CC from the detecting electrode 20 is. Accordingly, when the electrodes X.sub.1, X.sub.2, - - - X.sub.N or Y.sub.1, Y.sub.2, - - - Y.sub.M are supplied successively with the voltage V.sub.CC during each period of the pulse from the shift register 31 or 33 and the capacitance C.sub.P varies every period of the pulse from the shift register 31 or 33 in such a manner as shown in FIG. 3, the voltage V.sub.O at the detecting electrode 20 varies in stepwise manner every constant period T.sub.O as shown in FIG. 4 and takes the maximum value when the voltage V.sub.CC is supplied to one of the electrodes X.sub.1, X.sub.2, - - - X.sub.N or Y.sub.1, Y.sub.2, - - - Y.sub.M which is most close to the detecting electrode 20.
Then, in response to the voltage V.sub.O at the detecting electrode 20 thus obtained, a position of the detecting electrode 20 on the position determining plate 10 in a X direction in which the electrodes X.sub.1, X.sub.2, - - - X.sub.N are arranged or a Y direction in which the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M are arranged in detected.
In the concrete, the voltage V.sub.O at the detecting electrode 20 is supplied through an amplifier to a resonant circuit and a signal S.sub.B having a constant frequency and the maximum amplitude at a time point delayed by a predetermined time from a time point at which the voltage V.sub.O takes the maximum value, as shown in FIG. 4, is obtained from the resonant circuit. This signal S.sub.B is supplied to a Schmitt trigger circuit and a signal S.sub.Z which falls to a low level from a high level when the signal S.sub.B crosses a predetermined positive level V.sub.S to be high from low and rises to the high level from the low level when the signal S.sub.B crosses the zero level to be negative from positive is obtained from the Schmitt trigger circuit. Then, a time T.sub.D having lapsed from an instant t.sub.1 at which the first electrode X.sub.1 or Y.sub.1 located at the starting end of the position determining plate 10 is supplied with the voltage V.sub.CC to another instant t.sub. 2 at which the signal S.sub.Z first rises, is detected by counting clock pulses with such a sufficiently high frequency as to have a cyclical period corresponding to, for example, one fortieth of the period T.sub.O by a counter during the period from the instant t.sub.1 to the instant t.sub.2.
Output data D.sub.E obtained from the counter at the instant t.sub.2, which represent the time T.sub.D, correspond to the position of the detecting electrode 20 on the position determined plate 10 in the X direction or the Y direction, and the position of the detecting electrode 20 on the position determining plate 10 can be detected in response to the output data D.sub.E of the counter.
In the above mentioned system, if output data D.sub.EX and D.sub.EY obtained from the counter at the instant t.sub.2 in the detecting period for detecting the position of the detecting electrode 20 on the position determining plate 10 in the X direction and in the detecting period for detecting the position of the detecting electrode 20 in the Y direction, respectively, are in linear relations with actual positions of the detecting electrode 20 on the position determining plate 10 in the X direction and the Y direction, as shown by absolute lines 1X and 1Y in FIGS. 5 and 6, respectively, the position of the detecting electrode 20 on the position detecting plate is to be detected accurately. However, according to practical measure, the data D.sub.EX and D.sub.EY are not in the linear relations but in nonlinear relations with the actual positions of the detecting electrode 20 on the position determining plate 10 in the X direction and the Y direction such as shown by curves 2X and 2Y in FIGS. 5 and 6, resepctively. The result shown by the curves 2X and 2Y in FIGS. 5 and 6 has been obtained under such conditions that the interval between the centers of each two adjacent electrodes X.sub.1, X.sub.2, - - - X.sub.N or Y.sub.1, Y.sub.2, - - - Y.sub.M is 4.0 mm, the width of each of the electrodes X.sub.1, X.sub.2, - - - X.sub.N is 1.2 mm, the wdith of each of the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M is 3.5 mm, the thickness of each of the transparent insulating layers 12 and 13 is 0.175 mm, the period T.sub.O is 31.25 microsecond and the frequency of shift pulses for the shift register 31 and 33 is 32 kHz.
As spparent from FIGS. 5 and 6, the data D.sub.EX and D.sub.EY become larger by .alpha. and .beta., respectively, than respective ideal values when the detecting electrode 20 is located at the center of each of the electrodes X.sub.1, X.sub.2, - - - X.sub.N and Y.sub.1, Y.sub.2, - - - Y.sub.M, and become smaller by .alpha. and .beta., respectively, than respective ideal value when the detecting electrode 20 is located at a position between each adjacent two of the electrodes X.sub.1, X.sub.2, - - - X.sub.N and Y.sub.1, Y.sub.2, - - - Y.sub.M. The maximum deviation .beta. of the data D.sub.EY representing the position of the detecting electrode 20 on the position determining plate 10 in the Y direction corresponds to about one fortieth of the interval between the centers of each adjacent two of the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.N, that is, about 0.1 mm and therefore is relatively small, and, on the other hand, the maximum deviation .alpha. of the data D.sub.EX representing the position of the detecting electrode 20 on the position determining plate 10 in the X direction corresponds to about one tenth of the interval between the centers of each adjacent two of the electrodes X.sub.1, X.sub.2, - - - X.sub.N, that is, about 0.3 mm and therefore relatively large. Accordingly, acurrate positional detection on the position determining plate can not be performed, especially, in the X direction.
The reason why the maximum deviation .alpha. of the data D.sub.EX is larger than the maximum deviation .beta. of the data D.sub.EY is that the space between each adjacent two of the electrode X.sub.1, X.sub.2, - - - X.sub.N is larger than the space between each adjacent two of the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M. Considering this only, it seems to be possible measures to increase the width of each of the electrodes X.sub.1, X.sub.2, - - - X.sub.N to be substantially identical to the width of each of the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M so as to narrow the space between each adjacent two of the electrodes X.sub.1, X.sub.2, - - - X.sub.N to be substantially identical to the space between each adjacent two of the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M in order to reduce the deviations of the data D.sub.EX from the ideal value. However, since the electrodes X.sub.1, X.sub.2, - - - X.sub.N must be arranged not to have a shielding effect to the electrodes Y.sub.1, Y.sub.2, - - - Y.sub.M, which are disposed on the surface more distant from the detecting electrode 20 than the surface on which the electrodes X.sub.1, X.sub.2, - - - X.sub.N are disposed, during the detecting period for detecting the position of the detecting electrode 20 in the position determining plate 10 in the Y direction, the width of each of the electrodes X.sub.1, X.sub.2, - - - X.sub.N can not be widened.
Further, it is also considered to be another measures for reducing the deviations of the data D.sub.EX to divide each of the electrodes X.sub.1, X.sub.2, - - - X.sub.N into plural individual electrodes with a narrow space between each adjacent two of them. However, in such a case, the number of the electrodes used for detecting the position of the detecting electrode 10 on the position determining plate 10 in the X direction is increased plural times, and as a result of this, the number of the switches 31.sub.1, 32.sub.2, - - - 32.sub.N connected to these electrodes and the number of the output terminals N.sub.1, N.sub.2, - - - N.sub.N of the shift register 31 have to be also increased plural times. This results in the disadvantage that the circuit configuration in the system is so complicated and the cost of the system mounts up.